Mechanical waves are waves that require a medium. This means that they have to have some sort of matter to travel through. These waves travel when molecules in the medium collide with each other passing on energy. One example of a mechanical wave is sound. Sound can travel through air, water, or solids, but it can't travel through a vacuum. It needs the medium to help it travel. Other examples include water waves, seismic waves, and waves traveling through a spring.
Hi Kelly, I'm now 28 weeks pregnant with twins and I first started feeling them move when I was around 15/16 weeks pregnant but this is my second pregnancy. In my first pregnancy (not twins) I didn't feel any movement until I was 24 weeks pregnant. I think everyone feels their babies at different times so don't worry about anything.
This means that you'll probably be able to identify baby kicks earlier than first-time mums can. You may start to notice your baby moving at between 16 weeks and 18 weeks. The first time you were pregnant, you'd have been more likely to notice movements later, at 20 weeks, or even after this. The fluttering that you feel in your tummy is called quickening. These first movements can feel like wind, but in your second pregnancy you'll probably be able to tell when it's the real thing.